Long period fiber grating is an important element in the optical communications market, and its main commercial applications include the use of a phase mask fabrication method to produce a grating on fibers, which serves as a band rejection filter, a mode converter, a band rejection filter or a gain equalizer in various applications. The long period fiber grating has its advantages of being an all-fiber device and exhibits very low loss when connected to fibers, a low insert loss and a low back reflection, etc. In addition, the long period fiber grating is highly sensitive to high temperature, curvature, twist, and load, and thus is applicable for the development of optical communications and sensors. However, the long period fiber grating produced by photolithography has different properties according to the different conditions of the manufacturing process and needs accurate control. Once a fiber is written, it cannot be recovered or revised anymore.
In view of the foregoing shortcomings, an external force type tunable simple-structured long period fiber grating becomes the key point for the development of long period fiber grating. Two U.S. patents regarding the long period fiber grating filters are described below:
The U.S. Pat. No. 6,282,341 B1 disclosed a tunable mechanically induced long-period fiber grating filter, and this filter comprises a pair of periodic mechanical structures (which could be a periodic structure plus a plate) for mounting the fiber inside, ridges of upper periodic and lower periodic structures being engaged and aligned with each other for directly applying pressure on the surface of a fiber as to define a grating by means of directly inducing perturbations in the refractive index caused by the photoelastic effect. By changing the pressure applied on the periodic structures and controlling the perturbations of the refractive index, the frequency spectrum (loss) can be tuned. With the design of the shape and arrangement of the periodic structures, the wavelength and spectral form can be changed and the polarization dependence loss can be eliminated. The idea of such patent resides on the photoelastic effect caused by the perturbations of the refractive index, which is a result of applying pressure on the fibers, and thus it is necessary to prevent the microbending. Besides, the fiber must be jacketed, or else it will have a lower efficiency and additional loss (1˜2 dB).
The U.S. Pat. No. 6,408,117 B1 also disclosed mechanically induced long period fiber gratings. Unlike the foregoing patent, it only has a periodic structure on one side, but the other side is attached to a plate made of an elastic material for mounting a fiber between both sides. Screws are used for fixing and adjusting the tightness. The purpose of using an elastic material for the plate resides on producing a continuous microbending mode under the condition of applying an even pressure on the fiber to support the periodic structure when both sides of the periodic structure are engaged in order to produce a fiber grating. The objective of tuning the frequency spectrum can be achieved by adjusting the tightness of the screws, the selection of the elastic material, and the design, arrangement and change of the periodic structure. The idea of such patent emphasized on the fiber grating produced by the microbending, but the fiber must be jacketed (wrapped with plastics having a low refractive index), otherwise the intensity, the reliability and the depth of the frequency spectrum (loss) will be unsatisfactory.